The need to control the velocity of an elevator is well known. Reference is had, for example, to assignee's U.S. Pat. No. 4,751,984 of Walter L. Williams, Donald G. McPherson & Arnold Mendelsohn entitled "Dynamically Generated Adaptive Elevator Velocity Profile" issued Jun. 21, 1988, as well as to the art cited therein.
As noted in the Williams et al. patent, automatic elevator operation requires the control of elevator velocity with respect to zero or stop, at the beginning and the end of a trip, to speeds therebetween, which minimize trip time while maintaining comfort levels and other constraints. The time change in velocity for a complete trip is termed a "velocity profile." Automatic elevator control further requires control of the distance travelled during a trip in order to accomplish a precision stop at the destination floor.
Certain velocity profile generation strategies may lead to control instabilities. A common strategy is to use a phase-plane control for precision stopping, wherein dictated velocity is a function of the distance to go to the landing. As the distance-to-go approaches zero, the slope of the velocity/distance curve approaches infinity (.infin.). Using linear control theory, it can be shown that the slope of the phase-plane curve represents the position error gain for phase-plane control and is proportional to position loop bandwidth. For the speed control loop to track the dictated velocity profile with stability, its bandwidth must be greater by a significant factor than the bandwidth of the position control loop.
One strategy for reducing the required bandwidth is to limit the slope of the phase-plane velocity versus position profile (position error gain) to a maximum value, such that the position loop bandwidth is sufficiently lower than the velocity loop bandwidth.
Generally, the torque producing capability of elevator motors may vary with speed due to motor current, voltage, and/or power limitations. If the drive is not capable of maintaining the acceleration limit under all conditions due to these torque limits, some means of reducing the acceleration (and hence torque) in the corresponding portions of the velocity profile must be provided without compromising operation of the drive at its limit or complicating the profile generation more than necessary.
To avoid, inter alia, these problems, in Williams et al. each segment of the velocity profile was generated at one of the limits constraining the system; viz., at maximum jerk, maximum acceleration, maximum velocity, maximum position or loop gain, or maximum motor torque. The acceleration portion of the velocity profile preferably was generated in an open loop manner, beginning with constant (maximum) jerk, transitioning to constant (maximum) acceleration after an acceleration limit is attained, and jerking out (negative jerk) at a constant rate to maximum (contract) velocity when the maximum velocity is nearly attained However, although Williams et al. represented a very substantial advance in the art, it also was subject to improvement, to which the present invention is directed. The disclosure of the Williams et al. patent is incorporated herein by reference.